Method of forming p-n junctions in semiconductor material and apparatus therefor



Oct. 8, 1957 F. KOURY 2,809,135

METHOD OF FORMING P-N JUNCTIONS IN SEMICONDUCTOR MATERIAL AND APPARATUSTHEREFOR Filed July 22, 1952 l I p I ..l.

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I NVENTOR mmm/c K0012) United States Patent METHOD OF FORMING P-N JUNCTIONS IN SEMI- CONDUCTOR MATERIAL AND APPARATUS THEREFOR FredericKoury, Lexington, Mass, assiguor to Sylvania Electric Products Inc., acorporation of Massachusetts Application July 22, 1952, Serial No.300,171

9 Claims. (Cl. 1481.5)

The present invention relates to semi-conductors, especially silicon andgermanium, and in particular to the formation of area junctions therein.Such junctions exist between portions of the semi-conductor which are ofopposite conductivity types, adjoining each other at a barrier and areeffective for rectification, photodetection, etc.

The formation of a single crystal of germanium, for example, withuniform conductivity of either P-type or N-type has been accomplished bydrawing a crystal out of a supercooled melt of germanium containing theproper amount of the desired type of impurity. In order to produce a P-Njunction in a crystal thus grown, one practice suggested is to add to amelt of one type of germanium a sufficient amount of impurity to convertthe melt to the opposite type during the crystal growing process. Thishas the effect of growing, as a single integral crystal, an electricaldevice having N-type and P-type conductivity portions at opposite sidesof a barrier. This method presents the difficulty that each time themelt is to be changed in conductivity type, the amount of impurity inthe molten material increases, thereby affecting the electricalproperties of the crystals pulled from the melt subsequently.Semiconductors are known to be greatly affected by amounts of impurityin parts per million of the pure semiconductor material, and each timethe predominating type of impurity is reversed by adding impurities, thetotal amount of impurities increases.

An object of the present invention is to devise novel methods of growingcrystals having P-N junction-s therein without the necessity ofcontaminating the melt, either once or repeatedly in order to effectrepeated reversals of conductivity type in a grown crystal having one ormore barriers therein. A further object is to devise a noval method forproducing either single reversals or plural reversals of conductivitytype in a single bodyof germanium.

In the illustrative embodiment of the invention described in detailbelow it is seen that two separate cavities are provided in a cruciblewhere one cavity contains germanium or other semiconductor such as asemiconductor of one conductivity type and another cavity contains thesemiconductor of opposite conductivity type; and a crystal growingarrangement is adapted to shift either a seed crystal or a partly growncrystal from either melt to the other, and back if desired. When this isdone, a germanium crystal having opposite conductivity types adjoiningat a barrier is produced. Such structure has particular value where thebarrier is rectifying in character; and in order to effect a conversionof the barrier from one which may be predominantly of high resistivityas may occur, the crystal is subjected to a prolonged annealingtreatment after which the efficient rectifying property of the barriersat the P-N junctions in the crystal is assured. The illustrativeembodiments of invention, in different aspects, are shown in theaccompanying drawings, wherein:

Figure 1 is a diagrammatic cross-section of apparatus ice illustratingthe preparation of a crystal having multiple conductivity types therein;

Figure 2 is a diagrammatic illustration, greatly enlarged, of a crystalhaving multiple junctions therein; and

Figure 3 is a diagrammatic illustration, greatly enlarged, of amodification of the crystal illustrated in Figure 2.

Referring first to Figure 1 there is shown apparatus suitable forgrowing crystals having multiple conductivity types therein. Thisapparatus includes a furnace having a wall 10 which is adapted to beevacuated and is suitably formed of quartz so as to be transparent; andthere is further a crucible 12 as of highly pure graphite surrounded bycoil 14 for heating the crucible either by induction or by heatdeveloped in the coil as a resistance element.

The apparatus includes a vacuum seal 16 such as an O-ring assembly,through which a rod 18 may be drawn axially and indexed. Shaft 18 has aplate 20 that may be indexed when removed from its fiat-walled guidepassage 22 so that an offset seed-crystal holder 24 may be moved eitherto cavity 12a as shown or cavity 1212 as indicated in the dottedposition. The furnace advantageously may be operated evacuated, but itmay contain a gas inert in respect to the semiconductor type produced.

Germanium of a very high degree of purity (strainfree and of highresistivity when cool) is charged into the two cavities of the crucible,one charge predominantly containing an acceptor impurity and the otherpredominantly containing a donor impurity, as designated in the drawingby P and N. Crystal holder 24 is lowered to bring crystal C intoengagement with the N-type melt, and a cable 26 connected to shaft 18 isgradually pulled to raise the crystal holder. This causes an N-typecrystal to grow gradually at the end of crystal C. By abruptly raisingthe crystal, as by lifting pulley 28 over which cable 26 extends, holder24 can be indexed to bring crystal C with its N-type lower end intovertical alignment with the P-type melt, as indicated by the dottedlines. Then the crystal is lowered into contact with that melt bylowering pulley 28 while plate 20 arrests holder 24 against swivelingaway from the P-type melt. After contact of the N-type crystal end withthe P-type melt is established, growing of the crystal is commenced,with a P-type portion growing, on an N-type crystal as a single integralcrystal. Before actual pulling of the P-type part is com menced, it isdesirable to pause until the N-type surface is brought up to thetemperature of melt P, to minimize the tendency of developing a frozenhigh-resistance joint. Some slight amount of the previously pulledN-type crystal may actually melt into the crucible at this time, butthis is of only slight concern when the volume of the melt in relationto this remelted portion of the crystal is appreciated.

The process is advantageously repeated many times, to produce manybarriers, either unevenly distributed as shown in Figure 2 or evenly asin Figure 3. Where only a thin N-type layer is formed between two largeP-type portions, a series of multiple barrier units 36 can be formed bycuts along the broken lines shown in Figure 2, appropriate terminalleads 30, 32, and 34 being formed to the thin central N layer and to thelarger P-type ends thereon. Naturally a thin P-type region canalternatively be formed if desired between two larger N-type regions.Where large P-type portions of the crystal alternate with large N-typeportions, a large number of area-rectifiers 38 may be produced bytransverse diamond-wheel cuts as represented by the broken lines inFigure 3.

The furnace is one well adapted to controlled reproduction ofarea-junctions or P-N barriers; but because it is a time-consumingprocedure in setting up each crystalgrowing operation, the foregoing isespecially advantageous not only because of the comparative immunity r 3of the melts from contamination, but also because of the number of unitsthat can be formed during each furnace set-up. The number of units inany one crystalpulling sequence .is limited by the height of thefurnace, the content of the crucibles, and the dimensions of the P and Nportions of the crystal grown.

Before the crystals in Figures 2 and 3 are subdivided it is quitedesirable to subject them to a prolonged heat treatment, as for example24 hours at approximately SSW-608 C. for germanium, for the purpose ofimproving the barriers, in respect to increased rectification efficiencyand reduced resistance of the barrier in the forward on low-resistancedirection of the current.

Germanium with appropriate acceptor and donor impurities is readily andeffectively processed as described; but other semiconductors as siliconare contemplated, and the successive melts are described as of the samematerial but in broad concept the melts may be of differentsemiconductor materials as of silicon and germanium that are of the samediamond-cubic type of crystal structure.

The foregoing represents an eminently effective method of producingsemiconductor devices having P-N barriers, and devices having multiplebarriers at any desired spacings along single crystal, such as junctiontype crystal amplifiers. Variations in detail and varied applicationwill occur to those skilled in the art, and accordingly the appendedclaims should be accorded broad interpretation, consistent with thespirit and scope of the invention.

This disclosure includes concepts more broadly claimed in copendingapplication Serial No. 237,001 filed July 15, 1951, by Edward N. Clarke,which application is assigned to the assignee hereof.

What I claim is:

l. The method of froming a P-N junction in a crystalline semiconductor,which includes the steps of providing in a furnace chamber a melt of asemiconductor containing predominantly an impurity effective to impartone conductivity type to the semiconductor, providing in said chamberanother melt of a semiconductor containing an impurity effective toimpart the opposite type of conductivity, pulling a crystal from one ofsaid melts, transferring instantly Within the existing furnaceatmosphere the then pulled crystal to the other melt, and pulling anadditional amount of crystal extending integrally from said first-pulledcrystal.

2. The method in accordance with claim 1 including the further step ofreturning directly the crystal with the second-pulled crystal portion tothe first melt after only a thin layer has been added in thesecond-mentioned melt, and pulling an additional crystal portion fromthe firstmentioned melt whereby to produce multiple closely spacedjunctions.

3. The method of forming a P-N junction in a semiconductor in accordancewith claim 1, including additionally the step of heat-treating themulti-part crystal grown at an elevated temperature and for a prolongedtime, thereby to improve rectification efficiency and reduce the forwardresistance of the PN junction.

4. The method of making electrical devices each having a rectifyingjunction Within a crystal body, including the steps of maintaining aP-type melt and an N-type melt in a common furnace enclosure,alternately transferring instantly within the existing enclosureatmosphere a crystal from the P-type semiconductor melt to the N-typesemiconductor melt and reversely, holding the crystal end in contactwith each melt sufficiently long to establish continuity of growth of asingle crystal and after such contacts pulling the crystal to addalternating portions of N-type semiconductor and of P-typesemiconductor.

5. The method of making electrical devices in accordance with claim 4wherein the pulled crystal is thereafter subdivided into units each ofwhich has portions of different conductivity types and providing anohmic contact to each such portion.

6. The method of producing electrical devices each having a rectifyingjunction therein, which includes the steps of maintaining multiple meltsof different conductivity types in a common furnace chamber, pullingadditions to a semiconductor crystal of alternating conductivity typesby transferring the crystal to said melts instantly and successivelywithin the existing furnace atmosphere, growing a crystal portion ateach melt, and adding ohmic connections to the different crystalportions.

7. The method according to claim 6 including the steps of alternatelyand repeatedly shifting the crystal directly from a melt of onesemiconductor type to a melt of the opposite semiconductor type, andsubdividing the resulting crystal into units each embodying portions ofdifferent semiconductor types.

8. The method according to claim 6 wherein the melts are of germaniumpredominantly containing, respectively, acceptor and donor impurities.

9. Apparatus for forming a P-N junction in a crystalline semiconductorwhich includes multiple crucible chambers for containing semiconductormelts each of a different conductivity type, means for mantaining themelts molten, a crystal holder, all the foregoing devices beingcontained in a furnace chamber, means for gradually raising the crystalholder, and means for successively positioning the crystal holder inalignment with each of said crucible chambers.

References Cited in the file of this patent UNITED STATES PATENTS2,631,356 Sparks et a1 Mar. 17, 1953 2,727,839 Sparks Dec. 20, 19552,750,262 Pfann June 12, 1956 2,753,280 Moore July 3, 1956

1. THE METHOD OF FORMING A P-N JUNCTION IN A CRYSTALLINE SEMICONDUCTOR,WHICH INCLUDES THE STEPS OF PROVIDING IN A FURNACE CHAMBER A MELT OF ASEMICONDUCTOR CONTAINING PREDOMINANTLY AN IMPURITY EFFECTIVE TO IMPARTONE CONDUCTIVITY TYPE TO THE SEMICONDUCTOR, PROVIDING IN SAID CHAMBERANOTHER MELT OF A SEMICONDUCTOR CONTAINING AN IMPURITY EFFECTIVE TOIMPART THE OPPOSITE TYPE OF CONDUC-